Carbon Nanotube Thin Film Supported by Nickel Nanotube Array as Supercapacitor Electrode with Improved Rate Capability

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1 Graduate Theses and Dissertations Iowa State University Capstones, Theses and Dissertations 2016 Carbon Nanotube Thin Film Supported by Nickel Nanotube Array as Supercapacitor Electrode with Improved Rate Capability Hosein Monshat Iowa State University Follow this and additional works at: Part of the Mechanical Engineering Commons Recommended Citation Monshat, Hosein, "Carbon Nanotube Thin Film Supported by Nickel Nanotube Array as Supercapacitor Electrode with Improved Rate Capability" (2016). Graduate Theses and Dissertations This Thesis is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Graduate Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact

2 Carbon nanotube thin film supported by nickel nanotube array as supercapacitor electrode with improved rate capability by Hosein Monshat A thesis submitted to the graduate faculty in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Major: Mechanical Engineering Program of Study Committee: Shan Hu, Major Professor Reza Montazami Robbyn Anand Jaime J. Juarez Chao Hu Iowa State University Ames, Iowa 2016 Copyright Hosein Monshat, All rights reserved.

3 ii DEDICATION Every challenging work needs self-efforts as well as guidance of elders especially those who were very close to our heart. My humble effort I dedicate to my sweet and loving Father, Mother & Dr. Shan Hu, whose affection, love and encouragement make me able to get such success and honor.

4 iii TABLE OF CONTENTS Page LIST OF FIGURES... v LIST OF TABLES... viii NOMENCLATURE... ix ACKNOWLEDGMENTS... x ABSTRACT... xi CHAPTER I: INTRODUCTION Supercapacitance Mechanism Electrostatic double-layer Pseudocapacitance Supercapacitors characterization techniques Cyclic voltammetry Electrochemical impedance spectroscopy Frequency analysis Cyclic charge and discharge Leakage current Carbon Nanotubes Current Collectors Advantages Electrophoretic Deposition CHAPTER II: EXPERIMENTAL AND SIMULATION CHAPTER III: RESULTS Constant-Current EPD vs. Constant-Voltage EPD: Experiment and Simulation Constant-Current EPD With Different EPD Current and Time... 27

5 iv 3. Cyclic Voltammetry (CV) and Rate Capability Ni-foil/f-CNT Ni-foam/f-CNT NiNT/f-CNT Electrochemical Impedance Spectroscopy Leakage current resistance Relaxation time Leakage Current Test Galvanostat Charge-Discharge, Energy and Power Density, and Cyclic Stability Combined Thermal Analysis and Fourier Transform Infrared Spectroscopy (FT-IR) 40 CHAPTER IV: SUMMARY AND CONCLUSIONS REFERENCES... 45

6 v LIST OF FIGURES Page Figure 1. Electric double layer capacitors charge storage mechanism [2]... 2 Figure 2. Reversible mechanisms of pesudocapacitance, A) Underpotential deposition, B) Redox pseudocapacitance C), Intercalation pseudocapacitance [4]... 3 Figure 3. Cyclic voltammetry, capacitor voltage and current sweeping vs. time Figure 4. CV plot for a non-ideal supercapacitor Figure 5. Sinusoidal Current Response in a Linear System Figure 6. Nyquist plot Figure 7. Bode plot of a supercapacitor Figure 8. Impedances in A) Series, B) Parallel Figure 9. A) Cyclic charge and discharge test and B) IR drop for a non-ideal supercapacitor Figure 10. Single-wall and Multi-wall Carbon nanotubes structure Figure 11. Conductive current collectors A) 2D and B) 3D Figure 12. Electrophoretic deposition of functionalized CNTs [57] Figure 13. Multiphysics models of the electrophoretic deposition of functionalized CNTs to nickel nanotube current collector Figure 14. EPD of f-cnt onto nickel substrate via A) Constant voltage EPD and B) Constant current EPD

7 vi Figure 15. Multiphysics simulation of electrical field and f-cnt particle distribution around nanotubes in constant voltage EPD Figure 16. Multiphysics simulation of the electrical field and f-cnt particle distribution around nanotubes in constant current EPD Figure 17. Electrical field distribution around individual NiNTs and B) SEM image of NiNT/f-CNT showing faster deposition of f-cnt on tips of NiNTs Figure 18. Microstructure of f-cnts coated on, A) Nickel nanotubes/1500x, B) Nickel-foam/1500x, C and D) Nickel-foil/1500x and 30000x Figure 19. SEM image showing f-cnt on A) NFL1040_5000x, B) NFL1040_30000x, C) NFM1540_500x and D) NFM1540_30000x Figure 20. SEM images of f-cnt on NiNTs for Set A, B and C Figure 21. SEM image showing f-cnt umbrella on NiNT array in NNT1540 and NNT Figure 22. Rate capability of Ni-foam/f-CNT Electrodes produced with different EPD conditions Figure 23. Rate capability of Ni-foam/f-CNT Electrodes produced with different EPD conditions Figure 24. Rate capability of Ni-foil/f-CNT Electrodes produced with different EPD conditions Figure 25. A) CV curve of NFL1040, NFM1540 and NNT1510 and B) Rate capability of NFL1040 (83 F/g at 1 mv/sec), NFM1540 (183 F/g at 1 mv/sec) and NNT1510 (200 F/g at 1 mv/sec) Figure 26. Nyquist plot for NFL1040, NFM1540 and NNT

8 vii Figure 27. Imaginary capacitance versus frequency for NFL1040, NFM1540 and NNT Figure 28. Leakage current test results for NNT1510, NFM1540 and NFL Figure 29. A) CCD curves of NNT1510 at different current densities, B) Four cycles of CCD for NNT1510 at 2.5mA, C) Ragone plot of sample NNT1510 and D) Cyclic stability of NNT1510 at scanning rate of 50 mv/sec Figure 30. Combined thermogravimetric analysis and FT-IR Spectroscopy of NNT Figure 31. IR gas analysis of NNT

9 viii LIST OF TABLES Page Table 1. Common Electrical Elements... 9 Table 2. EPD parameters for constant current - EPD Table 3. Calculated parameters based on Randles circuit

10 ix NOMENCLATURE CCD CNT CV ECD EDLC EIS EPD ESR f-cnt FT-IR FTO MWCNT NFL NFM NiNT/NNT PPY rgo SEM SWCNT TGA UGF Cyclic Charge and Discharge Carbon Nano-tube Cyclic Voltammetry Electrochemical Deposition Electric Double layer Capacitors Electrochemical Impedance Spectroscopy Electrophoretic Deposition Equivalent Series Resistance Functionalized Carbon Nano-tube Fourier Transform Infrared Spectroscopy Fluorine Doped Tin Oxide Multi-Walled Carbon Nano-tube Nickel Foil Nickel Foam Nickel Nano-tube Polypyrrole Reduced Graphene Oxide Scanning Electron Microscopy Single-Walled Carbon Nano-tube Thermogravimetric Analysis Ultrathin Graphite

11 x ACKNOWLEDGMENTS I would like to thank my committee chair, Dr. Shan Hu, and my committee members, Dr. Reza Montazami, Dr. Robbyn Anand, Dr. Jaime J. Juarez, and Dr. Chao Hu, for their guidance and support throughout the course of this research. In addition, I would also like to thank my friends, colleagues, the department faculty and staff for making my time at Iowa State University a wonderful experience. I want to also offer my appreciation to those who were willing to participate in my surveys and observations, without whom, this thesis would not have been possible.

12 xi ABSTRACT Functionalized carbon nanotube (f-cnt) thin films were deposited onto plain nickel foil, nickel foam and nickel nanotube (NiNT) arrays using electrophoretic deposition (EPD to fabricate a CNT-based supercapacitor. A two-dimensional multiphysics model of the EPD process was established to investigate f-cnt particles deposition on nickel substrates with constant voltage and constant current modes. Combined scanning electron microscopy and electrochemical analysis elucidate that the best rate capability is achieved when f-cnt thin films conformally wrap NiNTs and provide the fastest electron transport between active materials and current collectors. The effect of micro and nanostructured current collectors on the rate capability of CNT-nickel supercapacitors was studied by deposition of f-cnt on nickel foil, nickel foam as well as nickel nanotubes (NiNT). 51% capacity retention was calculated for NiNT/f-CNT when scanning rate was increased from 1 to 50 mv/sec in cyclic voltammetry test whereas, nickel foam/f-cnt and nickel foil/f-cnt kept 35% and 30% of their initial capacities in the same test. The specific capacitance of 200 F/g and capacitance retention of 90% after 1000 cycles was obtained for NiNT/f-CNT cell. Frequency analysis demonstrates a higher ability of NiNT/f-CNT to get polarized with the lowest dielectric relaxation time as small as sec. Keywords: Functionalized carbon nanotube, Nickel nanotube arrays, Nanostructured current collector, Multiphysics model, Electrophoretic deposition, Electrophoretic deposition, Supercapacitor, Dielectric relaxation time.

13 1 CHAPTER I: INTRODUCTION Like batteries, supercapacitors are energy storage devices. Stored energy in batteries is produced by chemical reactions however supercapacitors store energy by physical separation of electrical charges [1]. Supercapacitors can provide high levels of electrical power and offer long operating lifetimes. They can deliver more energy per unit mass (energy density) than electrolytic capacitors and also higher charge-discharge rate than batteries [1, 2]. Furthermore, supercapacitors enjoy longer cycle-life, experience no memory effect and are safer compared with batteries [1]. 1. Supercapacitance Mechanism Electrostatic double-layer and pseudocapacitance are the two primary distinguished mechanisms for charge storage in supercapacitors [1] Electrostatic double-layer In electric double layer capacitors (EDLC), electrical energy is electrostatically stored by separation of charges between the conductive electrode and the electrolyte (Fig.1) [2]. Separation charge distance in a double layer is on the order of angstroms. The double layer comprises the excess charge stored on the electrode surface and the charge stored in the Stern, Helmholtz, and diffuse layers. The Helmholtz double layer model is a simplified description of the true double layer structure. [1, 3]. This double layer is formed as ions from the solution adsorb onto the electrode surface. EDLCs are not limited to charge transfer kinetics. Therefore,

14 2 they can be charged or discharged much faster than commercial batteries, which means they have higher energy releasing rate (power density) than rechargeable batteries [1, 3]. In the Macroscopic level, the capacitance of an EDLC is given by following equation [3]: C = ε r ε 0 A/d (1) where ε r is the dielectric constant dielectric which is related to the materials properties, ε 0 is permittivity of free space, A is the surface area of active material which is in contact with the electrolyte, and d is the double layer thickness. Figure 1. Electric double layer capacitors charge storage mechanism [2] Pseudocapacitance Electron charge-transfer between electrolyte and electrode could occur due to the fast sequence of reversible faradaic redox or intercalation process, that phenomena are the basis of pseudocapacitance behavior (Fig.2) [4].

15 3 Figure 2. Reversible mechanisms of pesudocapacitance, A) Underpotential deposition, B) Redox pseudocapacitance C), Intercalation pseudocapacitance [4]. Pseudocapacitors that could undergo higher reversible surface redox reactions rate can store more charges per volume than regular double-layer capacitors [5]. Higher surface redox reactions rate increases both energy density of lithium-ion batteries as well as power density of supercapacitors [5]. 2. Supercapacitors characterization techniques There are different well-known tests to evaluate electrochemical properties of a supercapacitor such as cyclic voltammetry, electrochemical impedance spectroscopy, cyclic charge, and discharge as well as leakage current test, which are briefly explained in following sections Cyclic voltammetry Cyclic voltammetry (CV) plots electrochemical cell current variation while a linear voltage-ramp is swept over a voltage range as shown in Fig.3. Usually, voltage sweep is

16 4 repeated between two limit potentials during the CV test, each pair of opposite direction sweeps is called a cycle [6, 7]. Figure 3. Cyclic voltammetry, capacitor voltage and current sweeping vs. time. CV test could be done using two or three electrodes in the cell, employing three electrode setup makes it possible to study the one electrode while it is not going to be affected by the electrochemistry of other electrodes [8]. The three electrodes are: Working Electrode: the electrode which is being tested. Reference Electrode: The electrode which has a constant electrochemical potential. Counter Electrode: An electrode which completes the circuit and is an inert electrode. In two-electrode setup, the reference and counter electrodes should be connected to one side of the capacitor. The potential difference between the working and the reference electrode will be measured and recorded. Usually, a voltage scan rate between 0.1 mv/s to 1 V/s is used for CV test of a supercapacitor [7, 9]. Lower voltage scanning rate allows the side reactions tracking in CV plot. However, the test may take several hours to be done. Ideally, capacitors

17 5 should have the same charge storage capacity at different charge/discharge rates however in reality, due to the mass and charge transport limitations; they demonstrate lower capacitance at higher charge/discharge rates. Using a range of different scanning rates make it possible to calculate the rate capability which can present the capacitor s ability to retain its high specific capacitance (capacitance per mass of active material) at high charge/discharge rate. The rate capability can be calculated by dividing the capacitance obtained in the fastest scanning rate to the one obtained by the slowest scanning rate [8]. The working principle of capacitors dictates that more electron transport paths and faster mass transport of solution phase ions (for pseudocapacitors, electron transport between electrode and electrolyte) increase cell rate capability [6, 7, 8]. For an ideal supercapacitor, the CV plot would be a rectangle however for a non-ideal supercapacitor, equivalent series resistance (ESR) causes the slow rise in the current and rounds two corners of the rectangle (Fig.4) [6, 7]. Due to the electrical resistance in the electrode, contacts, and the electrolyte, real commercial supercapacitors suffer power-loss during charge and discharge. The sum of the electrical resistances that mentioned above is called equivalent series resistance.

18 6 Figure 4. CV plot for a non-ideal supercapacitor Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) is a standard method to measure the capacitance and the ESR of a non-ideal supercapacitor. Applying a small AC potential to an electrochemical cell and measuring cell current is the basis of electrochemical impedance spectroscopy [10]. Having different electrical elements in a circuit makes it difficult to use simple ohmic resistance, in these situations using the concept of impedance is highly preferred. Impedance shows the ability of the circuit to resist the flow of electrical current [11]. Usually, 1 to 10 mv AC signal is applied to the cell during EIS test, which is small enough to hold the system in linear/pseudo-linear condition [10]. For a linear system, current respond to an applied sinusoidal potential would be a sinusoid at the same frequency but shifted in phase (Fig.5).

19 7 Figure 5. Sinusoidal Current Response in a Linear System. Applying a sinusoidal potential as a function of time with the radial frequency ω and the amplitude E0, has the form: E = E 0 sin (ωt) (2) For a linear electrical system the current respond has the form: I = I 0 sin (ωt + ) (3) where I 0 is the max current amplitude and is the phase shift. The impedance of the system is calculated as [10]: Z(ω) = E(ω) = E 0 sin (ωt) = Z I(ω) I 0 sin (ωt+ ) 0 sin (ωt) sin (ωt+ ) (4) where Z 0 is the impedance magnitude. Using Euler relationship (Eq.5) the impedance could be represented as complex number as defined in Eq.6 [10]:

20 8 exp(j ) = cos( ) + jsin( ) (5) Z(ω) = E(ω) I(ω) = Z 0(cos + jsin ) (6) Using Eq.6 and plot the real part on X-axis and imaginary part on Y-axis, we get a "Nyquist Plot" as shown in Fig.6. In Nyquist plot, each point represents the impedance at one frequency. A vector from the origin to each point of a Nyquist plot has a length of Z and make an angle with X-axis which called phase angle [11]. The main shortcoming of Nyquist plot is that it is not possible to tell the frequency related to the each point by taking a look at data points. To overcome that disadvantage of Nyquist plot, we can use Bode Plot in which both Z and the phase angle are plotted on the y-axis versus log frequency on the x-axis (Fig.7) [12]. Figure 6. Nyquist plot.

21 9 Figure 7. Bode plot of a supercapacitor. For carrying out an electrochemical analysis of EIS data, an equivalent electrical circuit model (include common electrical elements such as resistors, inductors as well as capacitors) is employed to fit a mathematical model on discrete EIS data. The electrical equivalent circuit should be designed based on the physical electrochemistry of the system [12]. Voltage-current relationship and the impedance of some common electrical elements are listed in Table 1[12]. Table 1. Common Electrical Elements Element Current Vs. Voltage Impedance Resistor V = IR Z = R Capacitor Inductor I = C dv dt V = L di dt Z = 1 jωc Z = jωl

22 10 Arranging basics elements in parallel and series pattern and designing a multi-elements circuit, as shown in Fig.8, makes it possible to model an electrochemical cell [12]. Figure 8. Impedances in A) Series, B) Parallel. There are simple formulas to calculate the linear impedance of circuit elements in both parallel and series combination, as follows [12]: Equivalnet impedance in series: Z eq = N i=1 Z i { Equivalnet impedance in parallel: Z eq = 1 1 N i=1z i (7) For series resistor combination, both resistance and impedance go up. However, series capacitors increase the equivalent impedance while decrease the capacitance [10, 12]. As mentioned before, understanding the physical phenomenon such as electrolyte resistance, polarization resistance, charge transfer resistance, diffusion, and coating capacitance of the electrochemical cell is required to design an electrical equivalent circuit for EIS data analysis Electrolyte resistance Ionic solution resistance is an important parameter for the impedance analysis of an electrochemical cell, which depends on different parameters such as temperature, ionic

23 11 concentration as well as ions type [11, 13]. Electrolyte electrical resistance could be calculated using Eq.8: R E = 1 k L A (8) where k is the electrical conductivity of the solution (1/Ω), A is the area of a bounded electrolyte with the length of L Polarization resistance Polarization occurs when an overpotential is applied at the electrode-electrolyte interface without resulting in electron transfer. For example, a system that approaches the behavior of an ideally polarizable electrode would be an Au electrode with an alkane thiol monolayer coating at the surface. The monolayer prevents faradaic reactions (passivates the electrode) over a wide potential range. In contrast, an example of a non-polarizable electrode is a reference electrode (e.g., Ag/AgCl) for which an incrementally applied potential leads to a large increase in faradaic current. Reference electrodes are not easily polarized (moved away from their equilibrium potential). Polarization resistance is measured as the slope of the current-potential curve at low overpotential, and it is the same as charge transfer resistance. [13] Charge transfer resistance If the reaction which occurs on the surface of the electrode is a single, kineticallycontrolled electrochemical reaction, then the resistance that prevents the current from flowing through such a reaction is called charge transfer resistance. Charge transfer resistance depends on different parameters such as temperature, electrolyte ion concentration as well as potential [13].

24 Diffusion The impedance that created by diffusion is called Warburg impedance which depends on the potential perturbation frequency [10, 14]. Warburg impedance is larger at low frequencies since diffusing reactants have to diffuse farther. In Bode plot, the Warburg diffusion leads to 45 shift of the phase plot, and in a Nyquist plot that will show up as a straight line with the slope of 45 at lower frequencies [10, 13, 14] Constant phase element Non-ideal capacitors (commercial double-layer capacitors) act like a constant phase which is a capacitor with an impedance as defined below [10]: Z CPE = 1 (jω) α Y 0 (9) where Y 0 is the capacitance and α is an exponent that is equal to 1 for an ideal capacitor, and less than 1 for a constant phase element Frequency analysis To study dielectric relaxation time of supercapacitors (response of a dielectric medium to an external, oscillating electric field), C -frequency plot should be generated, where C is imaginary part of capacitance and can be calculated as follows [15]: Z(ω) = Z real (ω) + j Z imaginary (ω) (10) C (ω) = Z real(ω) ω Z(ω) 2 (11) where Z(ω) is the impedance of the electrochemical system and ω is pulsation. Real and imaginary values of impedance are x and y axis of Nyquist plot, respectively. The relaxation time constant correspond to a transition of the resistive behavior of the supercapacitor to

25 13 capacitive behavior is calculated by1/f 0, where f 0 is pick-frequency of C - frequency plot. Relaxation time constant could also be presented by following equation [16]: τ 0 = R C 0 (12) where R is ESR and C 0 is specific capacitance considering the mass of active materials. Large relaxation time means that the charges which are electro-absorbed onto the electrode surface cannot be completely extracted from the electrode during discharge. There is irreversible charge-discharge happening when the charge/discharge frequency (as determined by the scan rate and the upper charge limit) is higher than the relaxation frequency (f 0 ) of the electrode. As a result, it is expected to see a better rate capability for the sample which has lower relaxation time Cyclic charge and discharge Cyclic Charge-Discharge (CCD) is a common test to investigate the performance and cycle-life of supercapacitors and batteries, which usually done at a constant current. Each loop of charging and discharging is called a cycle (Fig.9.A). Usually, repetitive cycles of CCD follows capacitance drop, the number of cycles indicates the cycle-life of the capacitor (cyclic stability), which is in the order of hundreds of thousands of cycles for commercial capacitors [17, 18].

26 14 Figure 9. A) Cyclic charge and discharge test and B) IR drop for a non-ideal supercapacitor. For an ideal capacitor, the slope of the CCD curve is constant and is defined by: dv dt = I C (13) where I is the constant current of CCD test and C is the cell capacitance. The shape of CCD curve can change to an exponential form by increasing the capacitor self-discharge. Moreover, a large voltage drop at each half-cycle occurs if the cell has a large ESR, which significantly reduces power density and capacitance of the cell (Fig.9.B) [17, 18]. The energy and power density are two important characteristics of each supercapacitor, which indicate how much energy can be released by a cell and how fast it can be achieved, and are defined by Eq.14 and Eq.15 [17]: E = CV 2 /2 (14) P max = V 2 /(4 m R drop ) (15) R drop = V drop /I (16)

27 15 where C is specific capacitance (F/g) obtained by discharging curve at different currents, V is the voltage of discharging, R drop is equivalent series resistance calculated from IR drop and m is the mass of active material. Plotting power density versus energy density (called Ragone plot) can give a performance comparison of different supercapacitors [17, 18] Leakage current A non-ideal charged capacitor cannot hold its constant voltage unless it is supplied by a constant current called leakage current that can be modeled as a resistance in parallel with the capacitor [19]. Current leakage results in self-discharging of a supercapacitor and reflects the flow of current when the cell is charged. Hence, lower leakage current will promote the supercapacitors performance. Leakage current can be measured in at least two ways [19, 20]: Measure the required current to maintain a specific voltage limit while a DC voltage is applied to the capacitor. Put a charged capacitor in open circuit condition and measure the voltage change during self-discharge. 3. Carbon Nanotubes Carbon nanotubes (CNTs) have been studied recently as a novel electronic and electrochemical material. Carbon nanotubes are divided into single (SWCNT) and multiwalled carbon nanotubes (MWCNT), based on the rolling graphite sheet structure, as shown in Fig.10 [21]. SWCNT structure is like wrapping a one-atom-thick layer of graphite called graphene into a seamless cylinder with the average diameter of nm and the average length of 1-50 µm [22]. SWCNTs Behave as a semiconductor can easily be modified to tailor

28 16 properties as an ink have high heat conductivity and the yield strength higher than carbon steels [23]. Figure 10. Single-wall and Multi-wall Carbon nanotubes structure. MWCNTs could be considered as more than two concentric rolled-graphene layers which have an approximate distance of 100 nm and the average diameter of 2-30 nm [22]. CNTs can show outstanding application as both electrodes and transducer components in biosensors [24]. Aligned MWNT grown on the platinum substrate could be used for the development of an amperometric biosensor. The opening and functionalization by oxidation of the nanotube array allow for the efficient immobilization of the model enzyme, glucose oxidase [24]. Due to the low electrical resistivity, high electrolyte accessibility, high charge transport capability, and high specific surface area of CNTs, they could be considered as attractive electrode materials for high-performance supercapacitors [25, 26]. Functionalized CNT [27-30], carbon composite of CNT [31, 32, 33], and nanocomposite of CNT with electrochemically active materials [34-47] have been proposed as supercapacitor electrode materials.

29 17 Peng et al. used chemical oxidation method to polymerize polypyrrole (PPy) onto reduced graphene oxide/carbon nanotube (rgo/cnt) which shows specific energy and power density of 14 Wh kg -1 and 6.62 kwkg -1 [48]. He et al. fabricated nitrogen-doped carbon nanotubes (N-CNT)/carbon foam (CF) employing chemical vapor deposition method, as a flexible supercapacitor [49]. N-CNTs/CF cell exhibited a power density of 69.3 kw kg -1 and the capacitance retention of above 95% after 5000 cycles at 50 A g -1 [49]. Yi et al. developed a supercapacitor CNT/nickel oxide nanosheets core-shell, porous carbon polyhedrons (PCPs) and aqueous KOH solution as a positive electrode, negative electrode, and electrolyte, respectively [33]. CNT/NiO/PCP cell shows a high specific capacitance of 996 (F/g) at 1 A g - 1, exhibits maximum energy density of 25.4 Wh kg -1 at a power density of 400 W kg -1 and the capacitance retention of 93% after 10,000 cycles [50]. In the case of graphene composite electrodes, amidation reaction was used to synthesize a graphene CNT composite with lamellar structure [51]. By making complementary bonding between the graphene oxide and CNTs prevent the agglomeration of CNTs [51]. 4. Current Collectors Advantages It has been well established in battery and supercapacitor research that the use of current collectors and highly conductive active materials will lower the overall resistance of the electrode and improve electron transport (Fig.11). Micro-device components assembly on a conductive 3D framework is state of the art approach for fabricating micro-devices with threedimensional architecture [52]. Using 3D porous current collectors reduces the total weight of the cell, allows a large mass of active materials to be deposited and makes strong current collector-active material binding [53].

30 18 Figure 11. Conductive current collectors A) 2D and B) 3D. Implementing conductive 3D current collectors (e.g. metal foams and nanostructures) provide advantages of low internal resistance rather than flat ones and accelerate electron transport [53, 54]. As an example, ultra-thin graphite thin film (UGF) acts as a highly conductive 3D framework network facilitates electron transport during charge and discharge of Ni(OH)2/UGF cell and [55]. 3D nanostructures afford increased interfacial area between electrode and electrolyte and ultimately improved rate capability of the CNT/nickel nanotube (NiNT) supercapacitor, provided that CNTs conformally coated on NiNTs. 5. Electrophoretic Deposition Electrophoretic deposition (EPD) can provide a conformal layer of active materials on the surface of an electrically conductive 3D current collector [53]. In this method, two conductive substrates are immersed with in an electrolyte; then an electric field is applies between the two electrodes to attract charged particles dispersed in the electrolyte towards a designated electrode (Fig.12). It worth to note that EPD can be carried out with constant current or constant voltage modes. Accumulation of charged particles, which are attracted towards electrode with opposite polarization, will result in the formation of a solid layer of dispersed

31 19 particles on the electrode surface. EPD has advantages of being scalable, low-cost, and solution-processed. Compared with other solution-based deposition methods, such as dropcasting, EPD provides higher controllability and is a promising technique to produce uniform and homogeneous microstructures [56]. Electrolyte conductivity is the main challenge of implementing EPD as an electrode fabrication method. Figure 12. Electrophoretic deposition of functionalized CNTs [57]. Pristine CNT is neutrally charged and non-polarized but functionalized CNTs (f-cnt) have functional groups such as carboxylate group which provides negatively charged particles of carbon nanotubes that can be uniformly dispersed in water. f-cnt can be uniformly dispersed into water to enable solution-based manufacturing of CNT electrodes such as EPD [58]. Appropriate functional groups can even introduce pseudocapacitance behavior and improve the overall charge storage capacity of CNT electrodes [59]. While most of existing studies on CNT-based supercapacitors have been focused on achieving high specific capacitance (F/g or F/cm 3 ) and impressive specific capacitance values have been reported, only limited work has been done to increase the rate capability of CNTbased supercapacitors. Current study aims to develop a CNT-based electrode with high specific

32 20 capacitance and rate capability. Using advantages of CNT, current collector, and nanostructured structures, a rational design of electrode is developed in the current study to achieve high capacity and rate capability. In current research, EPD is used to conformally coat f-cnt thin films onto nickel foil with a smooth surface (Ni-foil), nickel foam with micro-size pores (Ni-foam) as well as NiNT arrays with nano-sized surface roughness. CNTs are functionalized with carboxylic groups, which develop weak negative charge in water. Under an applied electric field, f-cnt particles holding negative charges are bonded with the conductive substrate by moving towards the positive electrode [58, 60, 61]. A uniform layer of f-cnt will be formed via f-cnt particles accumulating on the conductive substrate. The rate performances of the resulting three sets of electrodes are compared, and the structure-performance relationship is identified. Moreover, a 2D simulation has been done to study the difference between constant current and constant voltage EPD. Related experiments and simulation steps are explained in the next chapter.

33 21 CHAPTER II: EXPERIMENTAL AND SIMULATION In the current research, f-cnt was made by refluxing 0.1 g of pristine multiwalled CNTs in concentrated nitric acid and sulfuric acid (1:3 v/v) for 20 minutes at 110 o C. Corning filter with 0.22µm pore size was used to wash away acids and collect f-cnts. f-cnts were dried at 40 o C on a hot plate overnight and weighed mg/ml f-cnt solutions was made by dispersing f-cnts into deionized water with bath sonication (Branson 2510) for 1 hour. Electrochemical deposition (ECD) was used to grow nickel nanotube (NiNT) current collector. Specifically, Watt s nickel electroplating solution with 75 wt% of nickel sulfide, 15 wt% of nickel chloride and 10 wt% of boric acid in 200 ml of DI water was used as an electrolyte for ECD and Ni-foil was used as sacrificing electrode. Isopore track-etched polycarbonate template (Sigma Aldrich) with 13 mm diameter and 400 nm pore size was used as templates for nanotube growth. A gold seed layer was sputtered onto one side of the template followed by subsequent growth of a thin nickel film by ECD for 1 minute at 20 (V). Then the other side of the template was exposed to the nickel plating solution, and NiNTs were grown inside the pores of the template by ECD for 6 min at 1.5 volts. Finally, the filter was removed by chloroform to yield freestanding nickel nanotubes. Nickel foils, nickel foams and nickel nanotubes were coated with f-cnts by employing EPD for the time window between 5 to 15 seconds, current of 10 to 40 (ma) (Table 2) and using 0.05 mg/ml f-cnt solutions. Samples were divided into three sets based on the EPD

34 22 current value (Set A 5mA, Set B 10mA, and Set C 15 ma). For each EPD current, three different EPD times (10, 20 and 40 sec) were used. Table 2. EPD parameters for constant current EPD Sets Set A Set B Set C Ni-foil Current Collectors Ni-foam NiNT EPD Current (ma) EPD time (min) substrate substrate substrate NFL510 NFM510 NNT :10 NFL520 NFM520 NNT :20 NFL540 NFM540 NNT :40 NFL1010 NFM1010 NNT :10 NFL1020 NFM1020 NNT :20 NFL1040 NFM1040 NNT :40 NFL1510 NFM1510 NNT :10 NFL1520 NFM1520 NNT :20 NFL1540 NFM1540 NNT :40 Since f-cnts have weak negative charge, a positive polarization was applied to the nickel electrode and a negative polarization to the FTO glass as the counter electrode. The distance between the two electrodes was kept at 1.5 cm. The mass loading of f-cnt was determined by weighing the sample before and after f-cnt deposition with an analytical scale with 0.001mg readability. Also, thermogravimetric (TGA) analysis was performed on the f- CNT/NiNT electrode to confirm the mass loading of f-cnt. TGA was done in N2/O2 atmosphere between 40 to C with Netzsch STA449 F1 thermal analyzer. After EPD, the electrode was dried on the hot plate at 50 o C for 6 hours.

35 23 The microstructures of the electrodes were investigated by high resolution scanning electron microscopy (SEM; FEI Quanta 250). A two-electrode symmetric supercapacitor was fabricated with 1 M LiCl as the electrolyte for electrochemical measurements. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), leakage current and galvanostatic charge/discharge tests were done with Gamry Reference 3000 potentiostats/galvanostat for electrochemical characterization of the samples. To investigate the difference between the electrical fields made by constant current and constant voltage EPD, a 2D Multiphysics model was employed using COMSOL 4.5 as shown in Fig.13. Moreover, particle tracking of functionalized CNT particles was done using combined Electric Currents and Charged Particle Tracing physics models in COMSOL. Figure 13. Multiphysics models of the electrophoretic deposition of functionalized CNTs to nickel nanotube current collector.

36 24 CHAPTER III: RESULTS 1. Constant-Current EPD vs. Constant-Voltage EPD: Experiment and Simulation Fig.14 shows f-cnt deposition on NiNTs with constant-current EPD (10 ma for 40 seconds) and constant-voltage EPD (10 V for 40 seconds). It is clearly shown that constantcurrent EPD produced an overall uniform coating of f-cnt on the NiNTs. However, the constant-voltage EPD deposit a thick layer of f-cnt on NiNTs at the edge of the sample but sparse f-cnt coverage in central regions. Figure 14. EPD of f-cnt onto nickel substrate via A) Constant voltage EPD and B) Constant current EPD. To elucidate the reason for the observed difference in these two approaches, 2D Multiphysics models of the EPD processes were established via COMSOL 4.5. Multiphysics simulation results reveal that non-uniform distribution of electric field over the NiNT in constant voltage EPD is the main reason for the non-uniform f-cnt deposition. As can be seen in Fig.15, the electrical field at the edge of NiNT is much stronger than that in the center area,

37 25 and the f-cnt particle density around the edge of the sample is much higher than that in their central region, which agrees with experimental results. Figure 15. Multiphysics simulation of electrical field and f-cnt particle distribution around nanotubes in constant voltage EPD. On the other hand, electrical field distribution is smooth and uniform for constantcurrent EPD and the densities of charged particle (i.e. f-cnts) at the edge and the central region are almost the same, as demonstrated in Fig.16.

38 26 Figure 16. Multiphysics simulation of the electrical field and f-cnt particle distribution around nanotubes in constant current EPD. Electrical field distribution along individual NiNT was also investigated with the Multiphysics model. Electrical field strength drops by moving from the tip of NiNT to the root (Fig.17.A). Hence, it is expected to see more f-cnts attracted to the tip of the nanotubes, which is in good agreement with SEM image of NiNT/f-CNT sample (Fig.17.B). Keeping the above results in mind, using shorter NiNTs is recommended for developing a uniform coating of f-cnts on individual NiNT. However, reducing the aspect ratio of the NiNT will also reduce interfacial area between NiNT and f-cnt and slower the charge transport. In this research, it is found that NiNT grown by 6-min of ECD provides satisfactory performance.

39 27 Figure 17. Electrical field distribution around individual NiNTs and B) SEM image of NiNT/f-CNT showing faster deposition of f-cnt on tips of NiNTs. 2. Constant-Current EPD With Different EPD Current and Time Fig.18 illustrates microstructure of the samples made by EPD, as can be seen, constantcurrent EPD produced overall uniform coating of f-cnt on the NiNTs (Fig.18.A). Using a broad range of EPD current and time make f-cnt deposition rate controllable, so that, f-cnts can cover the surface of Ni-foam without blocking pores (Fig.18.B). Moreover, f-cnts could be uniformly deposited onto the smooth surface of nickel foil using constant-current EPD (Fig.18.C and Fig18.D).

40 28 Figure 18. Microstructure of f-cnts coated on, A) Nickel nanotubes/1500x, B) Nickelfoam/1500x, C and D) Nickel-foil/1500x and 30000x. Fig.19.A and B illustrate f-cnts coating on a nickel plate, NFL1040, a uniform layer of functionalized carbon nanotubes is formed on the nickel substrate, using constant current EPD. By receding current collector surface, electrical field strength decreases due to the internal electrical resistance of the f-cnt layer. As a result, a thick layer of f-cnt reduces the ability to attract and hold electrical charges, rate capability, and current leakage resistivity. Therefore, it is strongly preferred to make a thin layer of f-cnt on the nickel to keep both

41 29 capacitance and rate capability. Fig.19.C and D show microstructure of Ni-foam/f-CNT electrode, NFM1540, a uniform f-cnt coating layer is formed on the nickel foam matrix but did not block the pores. Using Ni-foam will provide more surface area of active materials. Hence, it is expected to see a higher rate capability than Ni-foil/f-CNT electrode. Figure 19. SEM image showing f-cnt on A) NFL1040_5000x, B) NFL1040_30000x, C) NFM1540_500x and D) NFM1540_30000x. Fig.20 shows the SEM images of NiNT/f-CNT electrodes in all Sets. By increasing EPD time, highly conductive f-cnt bridges formed between nanotubes, which introduce more

42 30 electron transport paths, hence, better rate capability behavior is expected. The morphology of NiNT/f-CNT electrodes of Set B are similar to those in Set A. However; Set C is showing obviously different morphology from that of other samples. In addition to f-cnt bridges that are found in all other samples, SEM of NNT1540 shows large-area and continuous f-cnt thin film covering the nanotubes like an umbrella (Fig. 21). Figure 20. SEM images of f-cnt on NiNTs for Set A, B and C. In fact, the f-cnt umbrella started to appear in NNT1520 as isolated islands, and as the EPD time increases, these isolated islands develop into continuous thin film over a large

43 31 area in NNT1540. Combining the Multiphysics simulation results, it is proposed that the formation of the f-cnt umbrella is caused by the stronger electric field on the tip of the nanotube, which causes f-cnt particles to accumulate faster at the tip. The f-cnt particles first form a canopy on top of an individual nanotube. After prolonged EPD time, individual canopies are bridged together by newly deposited f-cnt particles to form a continuous thin film covering multiple NiNTs like an umbrella. It is expected that this large-area umbrella structure has a negative effect on the rate capability because it prevents the ions in electrolyte from accessing the electrode area located underneath the umbrella. Figure 21. SEM image showing f-cnt umbrella on NiNT array in NNT1540 and NNT Cyclic Voltammetry (CV) and Rate Capability 3.1. Ni-foil/f-CNT Rate capability of Ni-foil/f-CNT cells was calculated based on CV test for scanning rates of 1 to 50 mv/sec (Fig.22). In the best case, rate capability of 30% was obtained for NFL1040, however, for the NFL/f-CNT cells of set B and set C, the rate capability remains

44 32 almost 29%. During deposition of f-cnt on nickel foil, a thick layer of active material rapidly forms on the surface of the current collector. By receding current collector surface, the internal resistance of f-cnt attenuates electrical field strength around the surface of active material. Hence, forming a thick layer of f-cnts on the nickel substrate can decrease the ability to hold electrical charges. As a result, after formation of a thin f-cnt layer on the nickel foil, increasing the EPD time and current decreases electron transfer rate and attenuates the electrical field around the f-cnt layer, which causes lower rate capability. Figure 22. Rate capability of Ni-foam/f-CNT Electrodes produced with different EPD conditions Ni-foam/f-CNT Rate capability of Ni-foam/f-CNT is shown in Fig.23, as can be seen, increasing EPD time and current gradually increases rate capability, since, more active materials are bind to the nickel current collector by increasing EPD time and current. NFM1540 shows the highest rate capability of 35% among all Ni-foam/f-CNT cells. Considering the higher surface area of

45 33 Ni-foam than Ni-foil, it needs stronger electrical field (EPD current) or longer deposition time to form a layer of f-cnts as thick as Ni-foil/f-CNT electrode and rate capability plot may reach a plateau for longer f-cnt deposition time or higher EPD current which used in current study. Higher rate capability of Ni-foam/f-CNT than Ni-foil/f-CNT cells can be due to the higher effective mass loading of active material, higher ability to release stored electrical charges during discharging and higher leakage current resistance. Figure 23. Rate capability of Ni-foam/f-CNT Electrodes produced with different EPD conditions NiNT/f-CNT Fig.24 exhibits rate capability is overall increasing with higher EPD current for NiNT/f- CNT cells. The effect of EPD current on rate capability is more significant with lower EPD times: rate capability increases by 28.3% after increasing EPD current from 5 to 15 ma in 10- second EPD, whereas, it increases by 7.3% in same current range with 40 sec of deposition time. As can be seen, Set C (highest EPD current among three sets) shows the higher rate

46 34 capability. Increasing EPD current results in higher amount of f-cnt particles attracted towards the metal substrate for a fixed time window. Subsequently, there will be more f-cnt bridges formed between NiNTs, resulting in faster electron transport and better rate capability. NNT1510 shows the highest rate capability among all three samples in set C, which is 51% when scanning rate increases from 1 to 50 mv/sec, it has the lowest EPD time, however. Lower rate capability of NNT1520 and NNT1540 is due to the formation of umbrella structure by f-cnts at prolonged EPD time, which prevents the electrolyte to meet the current collector located beneath of umbrella layer (Fig.21). Figure 24. Rate capability of Ni-foil/f-CNT Electrodes produced with different EPD conditions. In summary, high EPD current and long EPD time increase the mass loading of f-cnt which has a positive effect on the rate capability of Ni-foil/f-CNT and NiNT/f-CNT cells initially. However, due to the anisotropic deposition rate, excessive f-cnt will eventually

47 35 cover the top of NiNTs and block mass transport pathway between the electrolyte and the roots of NiNTs, hence, decrease rate capability. In Ni-foil/f-CNT cell, excessive f-cnt could attenuate the electrical field around f-cnt layer and decrease both specific capacitance and rate capability due to increasing current leakage. Fig.25.A shows CV curve for NFL1040, NFM1540, and NNT1510, at the scanning rate of 50 mv/sec. Figure 25. A) CV curve of NFL1040, NFM1540 and NNT1510 and B) Rate capability of NFL1040 (83 F/g at 1 mv/sec), NFM1540 (183 F/g at 1 mv/sec) and NNT1510 (200 F/g at 1 mv/sec). For a supercapacitor with small equivalent series resistance (ESR), CV plot would be rectangular with straight sides. Hence, NNT1510 behaves more like an ideal supercapacitor

48 36 considering the shape of CV curve. Since the same type of active material is used to build all f-cnt-based electrodes for the current study, rate capability difference between three sets of electrodes should be addressed to the structure of current collectors in each category. 3D structures such as NiNT arrays make higher surface area and more efficient contact between nickel substrate and CNT particles, which lower leakage current and increase the rate capability (Fig.25.B). EIS test was employed to investigate frequency behavior and leakage current resistance of the samples. 4. Electrochemical Impedance Spectroscopy 4.1. Leakage current resistance Electrochemical impedance spectroscopy was done for the frequency range of 1 Hz to 1MHz and AC/DC voltage of 1 mv and 1 V respectively. EIS results of samples with the best rate capability in each set of nickel current collector/f-cnt cells (NNT1510, NFM1540, and NFL1040) are shown in Fig.26. Figure 26. Nyquist plot for NFL1040, NFM1540 and NNT1510.

49 37 An equivalent circuit was used to model EIS discrete data for all of the samples (Fig.26), considering the physical meaning of the elements used in the circuit, Rct shows the leakage current resistance in each case. Table 3 shows Rct value of NNT1510, NFM1540, and NFL1040, NNT1510 demonstrate higher Rct (5730 Ω) than NFM1540 (1340 Ω) and NFL1040 (322 Ω). Therefore, the leakage current of NiNT/f-CNT cell is lower than Ni-foam/f-CNT which also has a lower leakage current than Ni-foil/f-CNT. Furthermore, NNT1510 Nyquist plot is closer to an ideal supercapacitor, having a straight line inclined to y-axis at the lowfrequency region, which represents faster ion diffusion in the structure of NNT1510 [58] (Fig.26). Table 3. Calculated parameters based on Randles circuit. Parameters NFL1040 NFM1540 NNT1510 Rct-cathode (Ω) Rct-anode (Ω) Goodness of Fit Relaxation time The frequency behavior of NFL1040, NFM1540 and NNT1510 was investigated using complex capacitance. As can be seen in Fig.27, NNT1510 demonstrates the lowest relaxation time and it has the highest rate capability among other cells, which previously confirms by CV test in section 3. EIS result confirms that using 3D structures in current collector made more electrons transport paths, leads to lower electron transport resistance and relaxation time. Hence, higher rate capability can be achieved. To quantify the leakage current of NNT1510, NFM1540, and NFL1040, leakage current test has been done.

50 38 Figure 27. Imaginary capacitance versus frequency for NFL1040, NFM1540 and NNT Leakage Current Test A DC potential of 0.5 V applied to the selected samples under test, and currents variation were measured to study leakage current in each case. Fig.28 shows current drop versus time for NNT1510, NFM1540, and NFL1040. As can be seen, the leakage current of NiNT/f-CNT cell (3.8 µa) is lower than Ni-foam/f-CNT (4.79 µa) and Ni-foil/f-CNT (34.67 µa). These results confirm that using 3D current collector decreases electron transport resistance, hence, increase the supercapacitor performance and efficiency. These results also double confirm Rct variation trend which obtained by EIS results in 4.1. Charge-discharge behavior, energy density, power density and cyclic stability of NiNT/f-CNT NNT1510, as the cell which shows the highest performance among other samples, have been investigated.